1906.] Resistance of Mice to the Growth of Cancer. 181 



survived, and 19 presented tumours after 10 days (66 per cent.). The tumours in treated 

 animals were smaller and grew less rapidity than those in the normal mice. 



Experiment 62 Z. Fifteen mice received an injection of normal defibrinated mouse 

 blood. Ten days later they were inoculated in the right axilla with 0*02 gramme of a 

 tumour of transplantation 61, Series ii E. All the mice survived, and 10 presented 

 tumours after 10 days (67 per cent.). At the same time 15 normal animals were 

 inoculated in right axilla in the same way. Ten mice survived, and all presented 

 tumours after 10 days (100 per cent). The same relation was again observed between 

 the tumours in the two series. 



Experiment 61, ii E. Thirty-nine mice received 0'3 c.c. of normal defibrinated mouse 

 blood on the back. Seven days later they were inoculated in the right axilla with 

 0*02 gramme of a tumour of transplantation 60, Series X. Of the 38 mice surviving 

 after 10 days, 15 presented tumours (39 per cent.). At the same time 20 normal animals 

 were similarly inoculated. All the 15 animals surviving after 10 days presented tumours 

 (100 per cent.). The same difference in the size and rate of growth was observed in the 

 animals treated with blood. 



The difficulty of readily obtaining large numbers of young mice artificially 

 protected against carcinoma has been a hindrance to the studies of this kind. 

 We have depended on the selection of animals which had been negative to 

 repeated inoculations, or on collecting mice in which spontaneous absorption 

 had occurred. Both proceedings take up a large amount of time and lead to 

 experiments being conducted under unfavourable conditions, owing to the 

 greater suitability of young animals for growth (and therefore for control 

 experiments). By means of a preceding injection of blood, however, the initial 

 stages of the refractory state can be quickly induced in young mice, and the 

 condition then enhanced by suitable doses of tumour material. When once 

 the initial stages have been passed, there is little difficulty in increasing the 

 refractoriness. 



Experiments conducted in this way appear to show that normal mouse 

 serum (0'3 to 0*5 c.c.) free from corpuscles does not confer protection to 

 anything like the same extent, if at all. The blood-corpuscles, suspended in 

 saline after being freed from the serum by centrifuging, on the contrary, were 

 nearly as efficacious as full defibrinated blood. The blood was injected into 

 the dorsal subcutaneous tissue over the root of the tail. The inoculations 

 were then made into the axilla, either on one or both sides. The absence of 

 the same degree of protective action after the injection of serum appears to 

 show that it is not due to mere super-addition of protective substances 

 normally present in the blood of the mouse, but rather that the protection 

 results from the active intervention of the living tissues in dealing with the 

 injected corpuscles. 



Although young mice are much more susceptible to inoculation, no differ- 

 ence could be discerned between animals treated with the blood of young and 

 old mice respectively. 



